Southern California -- this just in

The killing of a Southern California yachting couple marks another grim milestone in the persistent problem of Somali pirates on the Horn of Africa.

Pirates hijacked the couple's boat off the coast of Oman last week, apparently killing them and another American couple on board Tuesday as U.S. forces tried to negotiate a rescue. Targeting a civilian couple is seen as an escalation by the pirates. Scott and Jean Adam, boaters who were based out of Marina del Rey, had been on the high seas for several years, adventuring and distributing Bibles.

Pirating has become an international issue in the last decade and has been difficult to stop, given Somalia's weak government.

During 2008 alone, the pirates are believed to have collected more than $50 million, according to a Los Angeles Times report from Africa at the time. In response to a spate of hijackings, including that of a Ukrainian ship carrying 33 battle tanks and a Saudi tanker with $100 million of crude oil, warships from North Atlantic Treaty Organization countries, including the U.S., and other navies have been patrolling the area since last year.

Pirates have extended their reach and their activity has intensified, according Times reporting from Africa ("Pirates rule Somali coast"). About eight years ago, several Somali warlords began attacking illegal fishing boats that had been plundering and polluting the nation's waters. In 2005, more than 800 illegal vessels from Kenya, South Korea, China and other nations were exploiting Somalia's coastline.

Then the attackers began targeting humanitarian vessels, including some from the United Nations' World Food Program. Today, the brazen pirates, calling themselves Somalia's self-appointed "coast guard," attack virtually anything that floats, including private luxury yachts and even a U.S. naval boat.

Foreign seafarers usually offer little resistance and make obedient hostages. Ransoms this year alone have topped about $30 million, officials estimate.

Friends said Scott Adam had previously discussed the dangers of piracy when navigating the Arabian and Red seas.